When I first started using iChat I was using a Logitech microphone that cost me less than $50.00 U.S. I think it was this one http://www.macmall.com/p/Logitech-Headsets/product~dpno~7339505~pdp.ecdaejj
. In fact I think that was the microphone I used for my first podcast appearance (shameless podcast plug
If that is a bit out of your budget there are some cheaper Logitech mics on the market. Some of them start at about $10.00 and seem to have similar specs to the mic that I used. http://www.macmall.com/s?rch&q=logitech&n=macmsc-search&cn=Head%20Phones&man=Logitech&includeImage=true
If you would prefer to use a desktop microphone there are some available there as well but I think they would be mire prone to picking up background noise.